Directional control valves for use in a hydraulic drive system of hydraulic machines such as hydraulic excavators and hydraulic cranes are grouped into the open center type and the closed center type. One example of conventional directional control valve apparatus including an open center directional control valve is shown in FIG. 11.
In FIG. 11, a housing 101 includes a spool 102 inserted therein and displacing (through its stroke) depending on an input amount by which a control lever unit is operated. A feeder port 103 connected to a hydraulic pump and a bypass port 105 connected to a reservoir port 104 are formed in the housing 101 around a central portion of the spool 102. When the spool 102 is in a neutral position, the bypass port 105 is fully opened, causing all of a hydraulic fluid from the hydraulic pump to flow into a reservoir. As the spool 102 is moved to the left or right on the drawing, an opening area of the bypass port 105 is reduced, enabling part of the hydraulic fluid from the hydraulic pump to be supplied to an actuator through a meter-in throttle 102a or 102b and a load port 109a or 109b. When the spool 102 is in a full stroke position, the bypass port 105 is blocked off, enabling all of the hydraulic fluid from the hydraulic pump to be supplied to the actuator.
Additionally, denoted by 107a, 107b are overload relief valves and 108 is a load check valve, these valves being parts which are usually necessary for constructing the directional control valve apparatus. A not-shown main relief valve is also attached to the valve apparatus.
In the valve apparatus using such an open center directional control valve, since the bypass port 105 is throttled to have an opening corresponding to the input amount of the control lever unit, the so-called bleed control under which an actuator is driven while bleeding part of the delivery rate of a hydraulic pump can be effected at start-up of the actuator. The bleed control does not abruptly change the delivery pressure of the hydraulic pump and therefore provides a good operation feeling free from shocks imposed on the actuator.
On the other hand, a closed center directional control valve is a valve in which a feeder port is fully closed when a spool is in a neutral position. When combined with a pressure compensating valve, the closed center directional control valve can supply a hydraulic fluid to an actuator at a certain flow rate corresponding to the valve opening area regardless of variation in load pressure. However, since the closed center directional control valve is designed to always supply the actuator with the hydraulic fluid at a flow rate corresponding to the valve opening area, the delivery pressure of the hydraulic pump tends to change abruptly and the actuator tends to move so quickly. In view of such a problem, JP, A, 7-63203 proposes a hydraulic drive system which enables the bleed control to be effected in a hydraulic circuit including the closed center directional control valve as with the case of using the open center directional control valve.
The hydraulic drive system disclosed in JP, A, 7-63203 comprises a variable displacement hydraulic pump, a plurality of actuators driven by a hydraulic fluid delivered from the hydraulic pump, a plurality of closed center directional control valves for controlling flows of the hydraulic fluid supplied to the plurality of actuators, a plurality of control lever units for driving the plurality of directional control valves to operate, a bypass line connected to a hydraulic fluid supply line of the hydraulic pump, a bleed valve for returning the hydraulic fluid delivered from the hydraulic pump to a reservoir when the plurality of directional control valves are in neutral positions, and a controller for controlling the bleed valve to have an opening corresponding to an input amount by which the plurality of control lever units are operated.
In the hydraulic drive system thus constructed, the bleed valve functions like a center bypass throttle because the bleed valve is controlled to have an opening corresponding to the input amount of the control lever units. Accordingly, an equivalent operation feeling to that obtained in the bleed control using the open center directional control valve can be obtained while using a closed center valve as the directional control valve, thus resulting in good operability.
In the above-mentioned related arts, the bypass port 105 of the open center directional control valve and the bypass line described in JP, A, 7-63203 are both used for connecting the hydraulic fluid supply line of the hydraulic pump to the reservoir. Therefore, those port and line are referred to together as "an unloading circuit" in this Description.